Digital amplifier, pulse width modulator thereof and method for reducing pop noise for the same

ABSTRACT

A pulse width modulator for use in a digital amplifier, includes a pop noise reducer for reducing pop noise by controlling a width and a phase of a pulse of a PWM signal output from the pulse width modulator, wherein the pop noise reducer contains: a PWM pulse register for storing a width and a phase values of a pulse of the PWM signal; and a pulse generator for outputting the PWM signal according to the values stored in the PWM pulse register. The pulse width modulator reduces pop noise generated when power supply to a digital amplifier is started and interrupted.

FIELD OF THE INVENTION

The present invention relates to a method for eliminating noise from anaudio electronic appliance, and more particularly to a method and systemfor reducing pop noise in a speaker connected to a digital amplifier.

BACKGROUND OF THE INVENTION

FIG. 1 is a block diagram of a conventional digital amplifier and FIG. 2is a graph showing a waveform output from a typical Pulse WidthModulation (PWM) modulator.

A digital amplifier, which is a device for amplifying an audio signal ina digital manner, includes a PWM modulator 100 and a PWM amplifier 200for amplifying a PWM signal. In the digital amplifier, an audio signalis converted to a PWM signal which is a kind of digital signal by thePWM modulator 100 and the PWM signal is amplified by the PWM amplifier200. Then, the amplified signal passes through a low pass filter 300, sothat the amplified signal is converted back to the original analogsignal.

In a general class-D amplifier, Pulse Code Modulation (PCM) audiosignals sampled in a digital manner are input to the PWM modulator 100.Then, the PWM signal modulated by the PWM modulator 100 is amplified inthe PWM amplifier 200, passes through the low pass filter 300, and isthen input to a speaker 400 in a form of an analog signal.

The reason why the digital PCM signal is converted to the PWM signal isthat the PWM signal has a constant magnitude (or height) and storesinformation on the original signal into the sample width thereof asshown in FIG. 2, whereas the PCM signal is sampled at a predeterminedtime interval and each sample contains information on the originalsignal in the sample size thereof. Because the PWM signal has a constantheight (magnitude), it can define two states of 0/1 or ON/OFF and canmore easily perform amplification and achieve high output by using anFET switching. The reason why amplifying the PWM signal is that thesignal itself from the PWM modulator 100 has too small an output powerwhich is insufficient to directly operate the speaker.

The PWM amplifier 200 generally includes a gate driver and a switchingelement. As the switching element, Metal Oxide Semiconductor FieldEffect Transistors (MOSFETs) suitable for high-speed switching of bigelectric currents are usually used, which include ‘N channel type’transistors and ‘P channel type’ transistors.

Further, the PWM amplifier 200 may be classified into a ‘half bridgetype’ amplifier and a ‘full bridge type’ amplifier according to thecircuit types. In the ‘half bridge type’ amplifier, two Field EffectTransistors (FETS) are vertically stacked and the connection pointbetween the upper FET and the lower FET is connected to the speaker 400through the low pass filter 300. The ‘full bridge type’ amplifieremploys a circuit including a load connected between two half bridgeoutput ports.

Therefore, the PWM amplifier 200 can be classified into an ‘NN halfbridge type’ amplifier and an ‘NN full bridge type’ amplifier using onlyN channel MOSFETs, and a ‘PN half bridge type’ amplifier and a ‘PN fullbridge type’ amplifier using both a P channel MOSFET and an N channelMOSFET.

When the amplified PWM signal is suddenly applied to the speaker, itgenerates pop noise. The pop noise refers to noise generated due toabrupt voltage change when an audio appliance is powered on or off andmay cause a user to feel unpleasant. As shown in FIG. 2, because the PWMsignal employs the pulse width modulation, abrupt voltage increase isinevitable when the PWM signal is initially applied. Such abrupt voltageincrease causes abrupt introduction of electric current to the speaker400. Therefore, in the class-D amplifier using the PWM modulator 100, itis an important technical subject to reduce the pop noise occurring whenthe power is applied.

In order to eliminate such pop noise, there has been a method in which aPWM signal is supplied before operation power is supplied to atransistor of the PWM amplifier and the PWM signal is continuouslysupplied for a predetermined period of time even after the power supplyto the transistor of the PWM amplifier is stopped. Korean Patent LaidOpen Publication No. 2004-0098925 discloses such a conventional methodin detail.

According to another conventional method for eliminating pop noise, aswitch such as a relay may be used. In this method, the relay is amechanical switch which allows the PWM signal to be transferred to thespeaker only after passage of some time from the time point when the PWMsignal is applied. However, the relay inevitably increases the price ofthe product equipped with the relay. Therefore, it is impossible for alow-priced audio amplifier to employ the relay.

SUMMARY OF THE INVENTION

Accordingly, the present invention has been made to solve theabove-mentioned problems occurring in the prior art, and an object ofthe present invention is to provide a solution for reducing pop noisegenerated when power supply to a digital amplifier is started andinterrupted.

In accordance with an aspect of the present invention, there is provideda pulse width modulator for use in a digital amplifier, comprising: apop noise reducer for reducing pop noise by controlling a width and aphase of a pulse of a PWM signal output from the pulse width modulator,wherein the pop noise reducer includes: a PWM pulse register for storinga width and a phase values of a pulse of the PWM signal; and a pulsegenerator for outputting the PWM signal according to the values storedin the PWM pulse register.

In accordance with another aspect of the present invention, there isprovided a method for reducing pop noise in a digital amplifier using apulse width modulator, the method comprising the steps of: (a) supplyingelectric power in order to drive the digital amplifier; (b) outputting aPWM signal having a controlled pulse width and controlled pulse phaseduring a predetermined time interval by the pulse width modulator afterbeginning of the supply of the electric power; and (c) outputting anormal PWM signal by the pulse width modulator after the controlled PWMsignal is output, wherein the step (b) is performed by setting a widthand a phase values of each PWM pulse register of the pulse widthmodulator.

In accordance with still another aspect of the present invention, thereis provided a digital amplifier employing a pulse width modulationscheme, the digital amplifier comprising: a PWM modulator for pulsewidth modulating an audio signal; a pop noise reducer connected to thePWM modulator for controlling a pulse width and a phase of a PWM signal;a PWM amplifier for amplifying the PWM signal and outputting theamplified PWM signal; a low pass filter for filtering the amplified PWMsignal and outputting an analog signal; and a speaker for reproducingthe analog signal, wherein the pop noise reducer includes: a PWM pulseregister for storing a width and a phase values of a pulse of the PWMsignal; and a pulse generator for outputting the PWM signal according tothe values stored in the PWM pulse register.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will be more apparent from the following detailed descriptiontaken in conjunction with the accompanying drawings, in which:

FIG. 1 is a block diagram of a conventional digital amplifier;

FIG. 2 is a graph illustrating an output waveform of a typical PWMmodulator;

FIG. 3 is a block diagram of a PWM modulator and a pop noise reducer inaccordance with the present invention;

FIG. 4 is a block diagram of the pop noise reducer in accordance withthe present invention;

FIGS. 5A to 5C are graphs illustrating an output waveform of the PWMmodulator for reducing pop noise when an output port of the PWMmodulator is connected to a PWM amplifier of a ‘PN half bridge type’ ora ‘PN full bridge type’ using both a P channel MOSFET and an N channelMOSFET;

FIG. 6 is a graph showing an exemplary output waveform of the PWMmodulator for reducing pop noise when the output port of the PWMmodulator is connected to the PWM amplifier of an ‘NN full bridge type’using only N channel MOSFETs.

FIG. 7 is a graph showing an exemplary output waveform of the PWMmodulator for reducing pop noise when the output port of the PWMmodulator is connected to the PWM amplifier of an ‘NN half bridge type’using only N channel MOSFETs;

FIG. 8A is a graph showing pop noise measured when a typical PWM signalis applied to the digital amplifier; and

FIG. 8B is a graph showing pop noise measured when the PWM signal shownin FIG. 7 is applied to the digital amplifier.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, a preferred embodiment of the present invention will bedescribed with reference to the accompanying drawings. In the followingdescription and drawings, the same reference numerals are used todesignate the same or similar components, and so repetition of thedescription on the same or similar components will be omitted.

Hereinafter, a digital amplifier capable of reducing pop noise inaccordance with the present invention will be described in detail withreference to FIGS. 3 through 8B.

FIG. 3 is a block diagram of a PWM modulator 100 and a pop noise reducer150 in accordance with the present invention. The pop noise reducer 150in combination with the PWM modulator 100 can generate a PWM signalhaving a waveform which a designer of the PWM modulator 100 wants toobtain. Although the pop noise reducer 150 and the PWM modulator 100 areshown in separate blocks in the drawing in order to facilitate thedescription, it goes without saying that pop noise reducer 150 may beincorporated into the PWM modulator 100 as a part of the modulator 100.

FIG. 4 is a block diagram illustrating an example of the structure ofthe pop noise reducer 150. The pop noise reducer 150 includes a PWMpulse register 152, a pulse generator 154, an amp control register 156and an amp control signal generator 158. By using the PWM pulse register152 and the pulse generator 154, the pop noise reducer 150 can control awidth and a phase of each pulse of a PWM plus signal and a PWM minussignal. Further, the pop noise reducer 150 can also control the PWMamplifier 200 to start or to stop by using the amp control register 156and the amp control signal generator 158.

In detail, the PWM pulse register 152 includes PWM plus registers 0 ton, PWM minus registers 0 to n, a minimum pulse period register, a softstart period register, a soft stop period register and a weightingregister, which will be described later. The pulse generator 154includes a plus pulse generator and a minus pulse generator and each ofthe plus and minus pulse generators has a temporary register, a counter,a controller and a pulse generator, which will be also described later.

Each of PWM plus registers 0 to n stores a width and a phase values of apulse corresponding to each interval of the PWM plus signal. That is, aPWM plus register m (wherein m is an integer larger than or equal to 0and smaller than or equal to n) stores a width and a phase values of apulse in the (m+1)th interval of the PWM plus signal.

First, a width and a phase values in the PWM plus register 0 are storedin the temporary register of the plus pulse generator. The pulsegenerator of the plus pulse generator outputs the first interval of thePWM plus signal corresponding to the phase value of the temporaryregister. The controller compares a counting value of the counter withthe pulse width value of the temporary register. If the two values arenot identical, the counter simply adds a predetermined increment to thecounting value without any other operation. However, if the two valuesare identical, the counting value is reset to 0; a width and a phasevalues in the PWM plus register 1 are stored in the temporary register;and the second interval of the PWM plus signal corresponding to thephase value of the temporary register is output. Thereafter, the sameoperation as described above is repeated until output of the (n+1)-thinterval of the PWM plus signal is completed.

In the same manner as for the PWM plus signal, the PWM minus signal isoutput according to the values set in the PWM minus registers 0 to n.

In this way, a designer can produce a PWM modulator which can generate acontrolled PWM signal by setting a width and a phase values of each ofthe PWM pulse registers in the pop noise reducer 150. The specificconstruction of the pop noise reducer 150 may be modified in variousways by those skilled in the art.

An output port of the PWM modulator 100 may be connected to a PWMamplifier 200 of various types including an ‘NN half bridge type’ and an‘NN full bridge type’ using only N channel MOSFETs, and a ‘PN halfbridge type’ and a ‘PN full bridge type’ using both a P channel MOSFETand an N channel MOSFET. According to the type of the PWM amplifier 200,the PWM signal causing a minimal pop noise may have various waveforms.Therefore, it is necessary for the designer of the PWM modulator to findan optimum waveform capable of reducing the pop noise by controlling theoutput waveform from the PWM modulator 100.

In order to reduce the pop noise produced in a digital amplifier, it isnecessary to apply a PWM signal controlled during a predetermined timeinterval from the time point when power begins to be supplied or thepower supply is interrupted. Especially, the control during apredetermined time interval form the time point when power begins to besupplied is more important. After the predetermined time elapses, anormal PWM signal is output.

FIGS. 5A to 5C are graphs illustrating an output waveform of the PWMmodulator 100 for reducing pop noise in case that the output port of thePWM modulator 100 is connected to the PWM amplifier 200 of the ‘PN halfbridge type’ or the ‘PN full bridge type’ using both the P channelMOSFET and the N channel MOSFET. In this case, it is preferable to usethe minimum pulse period register, the soft start period register, thesoft stop period register and the weighting register in lieu of the PWMplus registers 0 to n and the PWM minus registers o to n.

In order to output the waveform as shown in FIG. 5A, the pop noisereducer 150 gradually increases the pulse width of the PWM signal whenthe power for the digital amplifier is supplied and gradually decreasesthe pulse width of the PWM signal when the power supply to the digitalamplifier is interrupted. At this time, the PWM plus waveform and thePWM minus waveform are symmetric structures wherein they have oppositephases and the same width. When the waveform as shown in FIG. 5A hasbeen applied, a remarkable reduction of pop noise was confirmed from afrequency analysis.

Referring to FIGS. 4 to 5C, the pop noise reducer 150 generates a pulsehaving a minimum width during a time interval M1 from the start of thepower supply for the digital amplifier, and then gradually widens thepulse width during a time interval S1. The time intervals M1 and S1 arecontrollable values and are set in the minimum pulse period register andthe soft start period register, respectively. A time interval A1, whichis the sum of the time interval M1 and the time interval S1, is aduration during which the pop noise reducer 150 performs the initialcontrol. The ‘PWM on’ interval is a time interval during which a normalPWM signal is output. In order to widen the pulse width during the timeinterval S1, there are two methods as follows: one is to widen the pulsewidth linearly as shown in FIG. 5B and the other is to widen the pulsewidth exponentially as shown in FIG. 5C. By setting the weightingregister of the PWM pulse register 152, the method of widening the pulsewidth during the time interval S1 can be determined.

In contrast, when the power supply is interrupted, the pop noise reducer150 gradually decreases the pulse width during a time interval S2, andthen generates a pulse having a minimum width during a time interval M2,and then stops the output of the PWM signal. The time intervals M2 andS2 are controllable values and are set in the minimum pulse periodregister and the soft stop period register, respectively. A timeinterval A2, which is the sum of the time interval M2 and the timeinterval S2, is a duration during which the pop noise reducer 150performs the final control. In order to decrease the pulse width duringthe time interval S2, there are two methods as follows: one is todecrease the pulse width linearly and the other is to decrease the pulsewidth exponentially. The method of decreasing the pulse width during thetime interval S2 is determined by setting the weighting register.

FIG. 6 is a graph showing an exemplary output waveform of the PWMmodulator 100 for reducing pop noise when the output port of the PWMmodulator 100 is connected to the PWM amplifier 200 of the ‘NN fullbridge type’ using only the N channel MOSFETs. As shown in FIG. 6, thephase of the PWM plus signal is opposite to the phase of the PWM minussignal and both signals have a constant pulse width. That is, the PWMplus signal and the PWM minus signal have a symmetrical waveform. Whenthe waveform as shown in FIG. 6 has been applied, a remarkable reductionof pop noise was confirmed from a frequency analysis.

FIG. 7 is a graph illustrating an exemplary output waveform of the PWMmodulator 100 for reducing pop noise when the output port of the PWMmodulator 100 is connected to the PWM amplifier 200 of the ‘NN halfbridge type’ using only the N channel MOSFETS. Table 1 shows a pulsewidth and a phase values stored in the PWM plus registers 0 to n and thePWM minus registers 0 to n of the PWM pulse register 152, whichgenerates the waveform shown in FIG. 7.

TABLE 1 Register Width Register Width name (ns) Phase name (ns) PhasePWM plus 640 0 PWM minus 640 0 register 0 register 0 PWM plus 1320 1 PWMminus 1320 1 register 1 register 1 PWM plus 975 0 PWM minus 655 0register 2 register 2 PWM plus 920 1 PWM minus 320 1 register 3 register3 PWM plus 1155 0 PWM minus 920 0 register 4 register 4 PWM plus 1245 1PWM minus 1155 1 register 5 register 5 PWM plus 1160 0 PWM minus 1245 0register 6 register 6 PWM plus 1065 1 PWM minus 1160 1 register 7register 7 PWM plus 1090 0 PWM minus 1065 0 register 8 register 8 PWMplus PWM minus 1090 1 register 9 register 9

When the PWM pulse register 152 is set with the values shown in Table 1,the waveform as shown in FIG. 7 can be obtained. In order to output thewaveform as shown in FIG. 7, the pop noise reducer 150 controls the PWMplus signal and the PWM minus signal to maintain the same phase whenpower is applied to the digital amplifier. After passage of apredetermined time interval, the pop noise reducer 150 changes theirphases to be opposite to each other and changes the pulse width to begradually increased.

In detail, when power is applied to the digital amplifier, i.e., ininterval 0, the PWM plus signal and the PWM minus signal are made tohave the same phase, i.e., LOW phase, and then, after a predeterminedtime is passed, i.e., in interval 1, both signals are changed to haveHIGH phase. Thereafter, after a predetermined time is passed, i.e., ininterval 2, both signals are changed to have LOW phase. Then, forexample, in interval 3 of the PWM minus signal, only the PWM minussignal is changed to have HIGH phase so that the phase of the PWM plussignal is opposite to the phase of the PWM minus signal. Referring toFIG. 7, interval 3 of the PWM minus waveform has a phase opposite to thephase of interval 2 of the PWM plus waveform. Thereafter, the bothsignals maintain their phases to be opposite each other and the pulsewidth of both signals are gradually increased.

FIG. 8A is a graph showing pop noise measured when a typical PWM signalis applied to the digital amplifier and FIG. 8B is a graph showing popnoise measured when the PWM signal shown in FIG. 7 is applied to thedigital amplifier. When a typical PWM signal is applied, irregularfrequencies from 2 to 20 KHz are observed. However, when the PWM signalshown in FIG. 7 is applied, frequencies having a smooth variance asshown in FIG. 8B are observed. Therefore, it is noted that the PWMinitial sequence as shown in FIG. 7 is very effective when the PWMmodulator 100 has an output port of an ‘NN half bridge type’.

A designer of the PWM modulator 100 can produce the waveform as shown inFIG. 7 by setting values of the PWM pulse registers of the pop noisereducer 150. However, the shown waveform is only an example, and thedesigner of the PWM modulator must select proper values according tovarious output ports connected to the PWM modulator 100.

A process of trial and error is necessary in order to find a PWM signalcausing a minimal pop noise for various output ports. Therefore, it ispossible to find a PWM signal causing a minimal pop noise by repeating aprocess of applying a certain PWM signal and then analyzing the popnoise. In order to analyze the pop noise, a method of analyzing thefrequency of the output signal from a digital amplifier is usually used.As a result of the analysis of the frequency, when the pop noise hasbeen remarkably reduced, the applied PWM signal is selected as a PWMinitial sequence of the corresponding output port.

Further, it is preferable to control the start and the stop of the PWMamplifier 200 as well as the waveform of the PWM signal in order toreduce pop noise. The amp control signal generator 158 of the pop noisereducer 150 generates an AMP_ENA signal according to values stored inthe amp control register 156. The amp control signal generator 158supplies the PWM amplifier 200 with the AMP_ENA signal in order to startor stop the PWM amplifier 200. For example, if the AMP_ENA signal ischanged from LOW values to HIGH values, the PWM amplifier 200 may beenabled and if the AMP_ENA signal is changed from HIGH values to LOWvalues, the PWM amplifier 200 may be disabled. By using the AMP_ENAsignal, it is possible to control the start timing or stop timing of thePWM amplifier 200.

One example of the AMP_ENA signal is illustrated in FIG. 7. Referring toFIG. 7, the AMP_ENA signal instructing the start of the PWM amplifier200 is applied to the PWM amplifier 200 when both the PWM plus signaland the PWM minus signal have the LOW phase initially, i.e., in interval0.

A process of trial and error is also necessary in order to find anAMP_ENA signal causing a minimal pop noise for various output ports.Therefore, it is possible to find an AMP_ENA signal causing a minimalpop noise by repeating a process of applying a certain AMP_ENA signaland then analyzing the pop noise.

According to the present invention, a PWM signal and an AMP_ENA signalhaving a predetermined waveform are applied to the digital amplifierwhen power supply to the digital amplifier is started and interrupted,so that the pop noise generated in the digital amplifier can be reduced.

Although a preferred embodiment of the present invention has beendescribed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

1. A pulse width modulator for use in a digital amplifier, comprising: apop noise reducer for reducing pop noise by controlling a width and aphase of a pulse of a pulse width modulation (PWM) signal output fromthe pulse width modulator, wherein the pop noise reducer includes: a PWMpulse register for storing a width and a phase values of a pulse of thePWM signal; and a pulse generator for outputting the PWM signalaccording to the values stored in the PWM pulse register wherein a PWMamplifier of one of a PN half bridge type and a PN full bridge typeusing both a P channel MOSFET and an N channel MOSFET is connected to anoutput port of the pulse width modulator and the pop noise reducergradually increases the pulse width of the PWM signal over at least twopulses when power is applied to the digital amplifier.
 2. The pulsewidth modulator as claimed in claim 1, wherein the pop noise reducergenerates a pulse having a minimum width during a time interval M1 froma beginning of power supply to the digital amplifier and graduallywidens the pulse width during a time interval S1, the time intervals S1and M1 being controllable values.
 3. The pulse width modulator asclaimed in claim 2, wherein the pop noise reducer gradually increasesthe pulse width of the PWM signal exponentially during the time intervalS1.
 4. A pulse width modulator for use in a digital amplifier,comprising: a pop noise reducer for reducing pop noise by controlling awidth and a phase of a pulse of a pulse width modulation (PWM) signaloutput from the pulse width modulator, wherein the pop noise reducerincludes: a PWM pulse register for storing a width and a phase values ofa pulse of the PWM signal; and a pulse generator for outputting the PWMsignal according to the values stored in the PWM pulse register, whereina PWM amplifier of an NN half bridge type using only N channel MOSFETsis connected to an output port of the pulse width modulator, the popnoise reducer controls a PWM plus signal and a PWM minus signal tomaintain the same phase when power is applied to the digital amplifier,and, after passage of a predetermined time interval, the pop noisereducer changes their phases to be opposite to each other during apredetermined time, and then, gradually increases the pulse widththereof while maintaining their phases to be opposite to each other. 5.A pulse width modulator for use in a digital amplifier, comprising: apop noise reducer for reducing pop noise by controlling a width and aphase of a pulse of a pulse width modulation (PWM) signal output fromthe pulse width modulator, wherein the pop noise reducer includes: a PWMpulse register for storing a width and a phase values of a pulse of thePWM signal; and a pulse generator for outputting the PWM signalaccording to the values stored in the PWM pulse register, wherein a PWMamplifier of an NN full bridge type using only N channel MOSFETs isconnected to an output port of the pulse width modulator, and the popnoise reducer controls a PWM plus signal and a PWM minus signal tomaintain the same phase when power is applied to the digital amplifier,and, after passage of a predetermined time interval, the pop noisereducer changes their phases to be opposite to each other during apredetermined time, and then, controls a PWM plus signal and a PWM minussignal to have a constant pulse width.
 6. A method for reducing popnoise in a digital amplifier using a pulse width modulator, the methodcomprising: (a) supplying electric power in order to drive the digitalamplifier; (b) outputting a pulse width modulation (PWM) signal having acontrolled pulse width and controlled pulse phase during a predeterminedtime interval by the pulse width modulator after beginning of the supplyof the electric power; and (c) outputting a normal PWM signal by thepulse width modulator after the controlled PWM signal is output, whereinstep (b) is performed by setting a width and a phase values of outputtedfrom a PWM pulse resister of the pulse width modulator, and in step (b),the pulse width of the PWM signal is gradually increased over at leasttwo pulses when an output port of the pulse width modulator is connectedto a PWM amplifier of a PN half bridge type or a PN full bridge typeusing both a P channel MOSFET and an N channel MOSFET.
 7. A method forreducing pop noise in a digital amplifier using a pulse width modulator,the method comprising: (a) supplying electric power in order to drivethe digital amplifier; (b) outputting a pulse width modulation (PWM)signal having a controlled pulse width and controlled pulse phase duringa predetermined time interval by the pulse width modulator afterbeginning of the supply of the electric power; (c) outputting a normalPWM signal by the pulse width modulator after the controlled PWM signalis output, (d) interrupting electric power supply in order to stopoperation of the digital amplifier; (e) outputting a PWM signal having acontrolled pulse width and controlled pulse phase by the pulse widthmodulator during a predetermined time interval after interruption of theelectric power supply; and (f) stopping output of the PWM signal by thepulse width modulator after the controlled PWM signal is output whereinstep (b) is performed by setting a width and a phase values outputtedfrom a PWM pulse resister of the pulse width modulator, and in step (e),the pulse width of the PWM signal is gradually decreased over at leasttwo pulses when an output port of the pulse width modulator is connectedto a PWM amplifier of a PN half bridge type or a PN full bridge typeusing both a P channel MOSFET and an N channel MOSFET.
 8. The method asclaimed in claim 6, wherein step (b) includes: (b1) generating a pulsehaving a minimum pulse width during a time interval M1 after beginningof power supply to the digital amplifier; and (b2) gradually increasinga pulse width during a time interval S1, the time intervals S1 and M1being controllable values.
 9. The method as claimed in claim 7, whereinstep (e) includes: (e1) gradually decreasing a pulse width during a timeinterval S2 after interruption of power supply to the digital amplifier;and (e2) generating a pulse having a minimum pulse width during a timeinterval M2, the time intervals S2 and M2 being controllable values. 10.A method for reducing pop noise in a digital amplifier using a pulsewidth modulator, the method comprising: (a) supplying electric power inorder to drive the digital amplifier; (b) outputting a pulse widthmodulation (PWM) signal having a controlled pulse width and controlledpulse phase during a predetermined time interval by the pulse widthmodulator after beginning of the supply of the electric power; and (c)outputting a normal PWM signal by the pulse width modulator after thecontrolled PWM signal is output, wherein step (b) is performed bysetting a width and a phase values outputted from a PWM pulse resisterof the pulse width modulator wherein, in step (b), when an output portof the pulse width modulator is connected to a PWM amplifier of an NNhalf bridge type using only N channel MOSFETs, a PWM plus signal and aPWM minus signal are initially controlled to maintain the same phaseduring a predetermined time interval and are then controlled to changetheir phases opposite to each other during a predetermined time intervaland are then controlled to increase the pulse width thereof whilemaintaining their phases to be opposite to each other.
 11. A method forreducing pop noise in a digital amplifier using a pulse width modulator,the method comprising: (a) supplying electric power in order to drivethe digital amplifier; (b) outputting a pulse width modulation (PWM)signal having a controlled pulse width and controlled pulse phase duringa predetermined time interval by the pulse width modulator afterbeginning of the supply of the electric power; and (c) outputting anormal PWM signal by the pulse width modulator after the controlled PWMsignal is output, wherein step (b) is performed by setting a width and aphase values outputted from a PWM pulse resister of the pulse widthmodulator, wherein, in step (b), when an output port of the pulse widthmodulator is connected to a PWM amplifier of an NN full bridge typeusing only N channel MOSFETs, a PWM plus signal and a PWM minus signalare controlled to maintain the same phase when power is applied to thedigital amplifier, and, after passage of a predetermined time interval,the pop noise reducer changes their phases to be opposite to each otherduring a predetermined time, and then, controls the PWM plus signal andthe PWM minus signal to have a constant pulse width.
 12. A digitalamplifier employing a pulse width modulation scheme, the digitalamplifier comprising: a pulse width modulation (PWM) modulator for pulsewidth modulating an audio signal; a pop noise reducer connected to thePWM modulator for controlling a pulse width and a phase of a PWM signal;a PWM amplifier for amplifying the PWM signal and outputting theamplified PWM signal; a low pass filter for filtering the amplified PWMsignal and outputting an analog signal; and a speaker for reproducingthe analog signal, wherein the pop noise reducer includes: a PWM pulseregister for storing a width and a phase values of a pulse of the PWMsignal; and a pulse generator for outputting the PWM signal according tothe values stored in the PWM pulse register wherein, if a PWM amplifierof a PN half bridge type or a PN full bridge type using both a P channelMOSFET and an N channel MOSFET is connected to an output port of thepulse width modulator, the pop noise reducer gradually increases thepulse width of the PWM signal over at least two pulses when power isapplied to the digital amplifier.
 13. The digital amplifier as claimedin claim 12, wherein the pop noise reducer further controls the startand the stop of the PWM amplifier by using an AMP_ENA signal whichinstructs the PWM amplifier to start or stop.
 14. A digital amplifieremploying a pulse width modulation scheme, the digital amplifiercomprising: a pulse width modulation (PWM) modulator for pulse widthmodulating an audio signal; a pop noise reducer connected to the PWMmodulator for controlling a pulse width and a phase of a PWM signal; aPWM amplifier for amplifying the PWM signal and outputting the amplifiedPWM signal; a low pass filter for filtering the amplified PWM signal andoutputting an analog signal; and a speaker for reproducing the analogsignal, wherein the pop noise reducer includes: a PWM pulse register forstoring a width and a phase values of a pulse of the PWM signal; and apulse generator for outputting the PWM signal according to the valuesstored in the PWM pulse register, wherein a PWM amplifier of an NN halfbridge type using only N channel MOSFETs is connected to an output portof the pulse width modulator, the pop noise reducer controls a PWM plussignal and a PWM minus signal to maintain the same phase when power isapplied to the digital amplifier, and, after passage of a predeterminedtime interval, the pop noise reducer changes their phases to be oppositeto each other during a predetermined time, and then, gradually increasesthe pulse width thereof while maintaining their phases to be opposite toeach other.
 15. A digital amplifier employing a pulse width modulationscheme, the digital amplifier comprising: a pulse width modulation (PWM)modulator for pulse width modulating an audio signal; a pop noisereducer connected to the PWM modulator for controlling a pulse width anda phase of a PWM signal; a PWM amplifier for amplifying the PWM signaland outputting the amplified PWM signal; a low pass filter for filteringthe amplified PWM signal and outputting an analog signal; and a speakerfor reproducing the analog signal, wherein the pop noise reducerincludes: a PWM pulse register for storing a width and a phase values ofa pulse of the PWM signal; and a pulse generator for outputting the PWMsignal according to the values stored in the PWM pulse register, whereina PWM amplifier of an NN full bridge type using only N channel MOSFETsis connected to an output port of the pulse width modulator, the popnoise reducer controls a PWM plus signal and a PWM minus signal tomaintain the same phase when power is applied to the digital amplifier,and, after passage of a predetermined time interval, the pop noisereducer changes their phases to be opposite to each other during apredetermined time, and then, controls the PWM plus signal and the PWMminus signal to have a constant pulse width.